When perpendicular recording is used for information storage on magnetic media, a soft magnetic underlayer (SUL) focuses magnetic flux from the write head (also called a transducer) into a recording layer. This enables higher writing resolution in the double layered perpendicular media with a SUL, compared to that in single layer perpendicular media without a SUL. The SUL material is magnetically soft with very low coercivity (less than a few Oersteds), and has high permeability. The saturation magnetization of the SUL needs to be large enough so that the flux from the write head can be entirely absorbed without saturating the SUL. Based on these requirements, appropriate head materials, such as permalloy, CoZrNb and FeAlN are chosen for the SUL.
However, from the playback (readback) performance point of view, the SUL is known to be a new noise source in addition to the recording medium noise due to the recording layer. This SUL generates noise which is attributed to domain wall motion in the SUL. The SUL noise is observed in the low frequency region in the readback spectrum and is also referred to as “spike noise” in the literature. An exemplary prior art SUL provides a large saturation magnetization, and softness of the SUL material, but no mechanism for suppressing domain wall formation and domain wall motion in the SUL is disclosed. The SUL is in a magnetically isotropic condition in the film plane. This structure is susceptible to magnetic perturbation from repetitive read/write operations during the lifetime of a HDD drive, and from strong magnetic fields existing in a HDD enclosure such as a spindle motor and a voice coil motor.
In order to suppress the SUL noise, complicated SUL fabrication processes and structure have been proposed. For example, domain wall pinning layers (CoSm) underneath a CoZrNb SUL have been used. The domain pinning layer can be anti-ferromagnetic such as IrMn, which requires an external magnetic field during the film deposition to bias the anti-ferromagnetic layer.
A large saturation magnetization of the soft magnetic underlayer is needed to ensure that the peak magnetic flux from the write head can pass through the soft magnetic underlayer without saturating the soft magnetic underlayer. With current-materials, if the thickness of the soft magnetic underlayer is increased to increase the saturation magnetization to the needed level by using a thicker underlayer or by adding additional multiple thick layers of SULs, then the surface roughness of the soft magnetic underlayer becomes so high that it is not compatible with the very narrow fly height of a high density read/write head. Readback noise is associated with the presence of domains in the soft magnetic underlayer, and this noise increases with increasing layer thickness and magnetization levels. Current low saturation moment soft magnetic underlayer arrangements are a barrier to increasing areal density in newer media designs.
A soft magnetic underlayer is needed that has a high saturation magnetization in combination with a low thickness and freedom from noise due to the presence of domains in the soft magnetic underlayer. Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.